FIG. 1 is a schematic view, in longitudinal section, of a conventional laminated core producing apparatus, and FIG. 2 is a plan view showing how a core sheet is formed. A strip 1 of a material for a laminated core product is guided to upper and lower metal dies 3 and 4 through guide rollers 2. When the upper metal die 3 lowers, the core sheet 1 is punched at first and second stations, as shown in FIG. 2; at a third station it is formed with crimp projections 11 or holes and at a fourth station contour punching-out and crimping are performed. Each time the upper metal die 3 moves up and down, the strip 1 is fed a predetermined pitch by the guide rollers 2, and with this operation repeated, punched out core sheets are laminated in the fourth station of a lower metal die 4. On the other hand, at a timing obtained by a microswitch 6 for detecting the up and down movement of the upper die 3, the thickness of the strip 1 is measured by a sheet thickness sensor 7 before it enters the metal dies, the data being successively shifted until the thickness-measured portion of the core sheet reaches the third station of metal die and being then stored in a control device 8. The control device 8 successively adds the sheet thickness at the third station of metal die, to find the sum. A punch 10 at the third station is movable by a solenoid 9, as shown in FIGS. 3 and 4. The control device 8 controls the solenoid 9 in such a manner that when said sum of sheet thicknesses is below a preset target value for the laminated core, the punch 10 is caused to project to form a crimping projection 11 on the strip 1, as shown in FIG. 3, but when said sum reaches the target value, it is caused to project to form a throughgoing hole, as shown in FIG. 4, whereupon said sum is set to zero. Then, the punched out core sheets 5 in the metal die at the fourth station are in an unbound state for the preset target value and become a laminated core product of predetermined thickness.
The usual laminated core product is formed by laminating 40-100 core sheets, and the preset value for lamination thickness is 20-50 mm in the case of a core sheet having a thickness of 0.5 mm. The dimensional accuracy of the product depends on lamination thickness, but increasingly high accuracy is being demanded year by year. For example, for a specification lamination thickness of 50 mm, it is 50.+-.0.5 mm or thereabouts, and if the nominal value of sheet thickness is 0.5 mm, 100 sheets will be assembled. In this case, the point of time when the control device 8 controls the solenoid 9 (referred to as the throughgoing hole making solenoid) to form a throughgoing hole is when the aforesaid sum is not less than the target value T, which is defined by the following formula. ##EQU1##
Substituting the lamination thickness preset value of 50 mm and the sheet thickness of 0.5 mm gives T=49.750 mm, and the total lamination thickness H of the laminated core product obtained is 49.75 mm.ltoreq.H&lt;50.25 mm.
However, since the allowable lamination thickness of the core product is 50 mm.+-.0.5 mm, as described above, the allowable lamination thickness measurement error is .+-.0.25 mm. Therefore, the sheet thickness measurement error for a single core sheet is obtained by dividing said value by 100, or .+-.0.0025 mm. That is, there is a need for a sheet thickness sensor 7 capable of measuring 0.5-mm thick core sheets with an absolute accuracy of .+-.2.5 .mu.m. A similar calculation shows that when the core product thickness specification is 100.+-.0.5 mm and the standard sheet thickness is 0.5 mm, the accuracy per sheet is .+-.0.00125 mm. Further, this sheet thickness sensor should have a quick response, capable of measurement while the core sheet 1 is being intermittently fed at high speed. However, there is no commercially available sensor which satisfies said accuracy, other than the contact type sheet thickness sensor adapted to be brought into direct contact with the core sheet. If the contact type sheet thickness sensor is used, however, because of contact between the sensor and the core sheet, prolonged use will result in the sensor being gradually worn, no longer assuring the accuracy. For this reason, periodical inspection is required at short intervals. Further, there are many instances where the specification cannot be satisfied in the matter of said response speed.
Thus, there has been a drawback with the conventional system that, at present, since the accuracy of the sensor is insufficient it has been impossible to perform lamination thickness control with high accuracy.